MCU enhances DC/DC converter output for hybrid/electric vehicle applications

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Concerns about climate change and gas prices have strengthened the market for eco-friendly vehicles such as hybrid vehicles (HVs) and all-electric vehicles (EVs). For propulsion, these vehicles use an electric motor powered by a high-voltage lithium-ion battery. For best efficiency, the powerful DC motor drive voltage is usually boosted to several hundred volts by a DC/DC inverter. Meanwhile, HVs and EVs typically use the same electronic control units (ECUs) as gasoline-powered vehicles for functions like body control and instrument clusters. That means they are designed to run on a 12-V power supply. As a result, the DC/DC converter also is used to step down the Li-ion battery’s few hundred volts of output to standard 12-V DC.

 

Figure 1: In an eco-friendly vehicle, an inverter boosts the output of a lithium-ion battery to several hundred volts DC (orange) to run the electric motor and also steps down the output to 12-V DC to run the ECU (light blue). The CANbus network is shown in dark blue.

Figure 1: In an eco-friendly vehicle, an inverter boosts the output of a lithium-ion battery to several hundred volts DC (orange) to run the electric motor and also steps down the output to 12-V DC to run the ECU (light blue). The CANbus network is shown in dark blue.

 

The ECU performs mission-critical tasks, so the DC/DC converter is essential to the safe operation of a vehicle. The converter also must be efficient. It needs to deliver a stable power supply to the ECU, even with any battery voltage fluctuations. The circuit design must be highly accurate, efficient, and compact enough for a space-constrained vehicle system. Above all, it must be reliable, since an ECU malfunction could lead to failure, damage, and even injury or death.

 

A DC/DC converter is only as good as its controller, however. New technology successfully reduces component count using the digital processing capabilities of an MCU. It flexibly and easily supports system requirements using the software platform. It also can send failure information to the driver via the vehicle network, which simplifies vehicle maintenance.

 

Digital advantage

To better understand how an MCU can impact the operation and functionality of a digital DC/DC converter, consider the Spansion MB91550. The MCU can enable two simultaneous control loops: for example, one for output voltage and one for the current (see figure 2). Running on a clock speed of up to 200 MHz, it can generate a highly accurate PWM waveform that is key to maintaining precise control. Phase-adjusting the cycle of PWM output of the converter allows it to perform power factor control (PFC). The MCU also supports the functional safety requirement defined by ISO 26262.

 

Figure 2: With the aid of a slope-compensation unit and a comparator, the MB91550 microcontroller can control a digital DC/DC converter to deliver a highly efficient system.

Figure 2: With the aid of a slope-compensation unit and a comparator, the MB91550 microcontroller can control a digital DC/DC converter to deliver a highly efficient system.

 

Let’s take a closer look at the process. A 12-bit A/D converter digitizes the voltage, then a 32-bit CPU with FPU sets a reference of comparator and a slope compensation value designed to buck the voltage down to the target level (see figure 3). This slope-compensation value set passes through a 10-bit D/A converter and is sent to the slope compensation unit of the current feedback loop. After processing by the CPU, the current passes to a comparator that checks for deviation from some preset reference value. Depending on the result, the converter adjusts the duty cycle of the PWM output for the present switching cycle. If the current is above the reference value that signifies an overcurrent condition, the device switches to the off state while revising the commanded PWM output at the same time.

 

Figure 3: The comparator compares inductor current against a reference value to determine how to modify PWM output parameters in order to deliver stable voltage.

Figure 3: The comparator compares inductor current against a reference value to determine how to modify PWM output parameters in order to deliver stable voltage.

 

The slope compensation circuit and dedicated comparator give the MCU the ability to directly control the PWM duty cycle of the DC/DC converter without waiting for the next duty cycle. This is a key benefit of the MCU—the converter can respond to rapid fluctuations in the actual load condition of the motor and monitor both current and voltage of the target system. The PWM output circuit supports a dead-time generator timer and phase-shift function for waveform control (see figure 4).

 

Figure 4: Block diagram shows a phase shift – full bridge DC/DC converter circuit.

Figure 4: Block diagram shows a phase shift – full bridge DC/DC converter circuit.

 

A look ahead

HVs and EVs are important elements of the eco-friendly society of the future. In this article, we focused on MCUs applied to vehicles, but in reality, that future is full of DC-based devices and systems like solar panels that require power management. The types of MCUs we discussed here can be applied to many such systems to improve efficiency, enhance performance, and ensure safety.

 

Figure 5: In the connected world of the future, security will be essential.

Figure 5: In the connected world of the future, security will be essential.

Microcontrollers can be the enablers not only for an efficient society but for a secure and safe one. To realize this vision, the digital electronics of the future need to rise to the occasion in a number of ways:

  • Everything will be connected in the near future. The vehicle is very important for transportation, but it also can be a threat if not properly protected from cyber attack. Digital components like MCUs need to be designed to protect vehicles from external cyber attacks (see figure 5).
  • Model-based development (MBD) can significantly reduce embedded software development time through automatic code generation. In this simulation environment, multiple safety hazards also can be tested even before having a real system. By evaluating system response to unusual events that may not occur in standard testing, a simulation environment can help with the development of a safe system.
  • Integration can reduce cost while optimizing performance.
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